This invention relates to a process for the production of lower polymers, i.e., dimers and trimers, from isobutene. More particularly, the invention relates to a fixed bed catalytic process.
The acid catalysis of the reaction of isobutene and water to produce t-butanol is well known. More recently, acid ion exchange resins have been used as the catalysts, e.g., British patent specification Nos. 1,309,164 and 1,396,488, where the reaction of propylene and water was illustrated in a fixed bed acidic ion exchange resin with various isobutene polymers produced as by-products.
Commercial operations would have to be continuous to be feasible. Fixed bed procedures were initially proposed as the most desirable because of simplicity of operation, i.e., isobutene is passed over the catalyst and hydrocarbon phase contain polymer recovered. The reaction is exothermic and temperature control has presented a problem. However, another problem which is a serious detriment is a rapid increase in by-product formation. At the beginning of the process, the reaction proceeds as expected. Side reactions begin and increase as the reaction proceeds. The side reactions include particularly polymerization, which is uncontrolled and from which results diisobutene, triisobutene, higher oligomers, and codimers of isobutene and n-butenes.
The formation of diisobutene and triisobutene is not particularly undesirable since these materials are of commercial value. The higher oligomers, however, are substantially waste and in some cases, are dark, gummy materials, which can foul equipment or otherwise interfere with the process. Furthermore, the uncontrolled side reaction is a detriment, since product distribution cannot be adjusted as desired. It should also be noted in the prior art that by-product selectivity could be reversed only by the cessation of the reaction, back-washing of the catalyst with water or replacement of the catalyst.
As can be seen in the prior art, the dimer and/or trimer products although valuable as gasoline octane improvers were the side benefit of the t-butanol process. The maximization of lower polymers has been prevented by the presence of water which favors the hydration reaction. The prior art used large excesses of water, i.e., 3 to 35 moles of water per mole of isobutene, mainly to control the reaction temperature. In a related application by the same inventors filed of even date herewith entitled: FIXED BED PROCESS FOR THE PRODUCTION OF T-BUTANOL, Ser. No. 799,103, filed May 20, 1977 a method of isobutene hydration using a fixed bed acidic resin catalyst and less than stoichiometric amounts of water is disclosed, i.e., a mole ratio of water:isobutene of less than 0.24 to 1:1. The temperature control in the hydration process and the present polymerization process is overcome in a novel and unobvious method. In fact, the two processes are completely compatible, in a single reaction system, i.e., reactor, catalyst, feed, with the desired product being obtained by adjustment of the water content of the feed to the reactor as taught in the respective applications.
In the present invention, dimerization of isobutene is the desired result. Dimerization is understood to be the addition of one molecule of isobutene to one other molecule of isobutene to produce a product having twice the molecular weight of isobutenes. Stated otherwise, a dimerization is the reaction of olefinic molecules to produce one-half the number of other olefin molecules. The process as described below is well adapted to the production of dimer, however, some trimer (a triple isobutene molecule) and higher products are produced as well as is some codimer, resulting because most isobutene streams also contain substantial amounts of n-butene. A codimer as the term is used herein is understood to be 1 molecule of isobutene reacted with a molecule of n-butene. The dimer is easily separated from the by-products if desired.
An advantage of the present invention is a continuous commercially adaptable process for producing isobutene dimer as the principal reaction product. Another advantage is that the present process suppresses the formation of by-product polymerizations. A particular feature of this invention is the coproduction of small amounts of t-butanol and the adjustability of the process as described to allow an increase or decrease in t-butanol as desired. It is also a feature of this invention that temperature control of the fixed bed reaction has been obtained. These and other advantages will be more apparent from the further discussion of the invention.
It is an advantage of the present invention that benefits of the fixed bed isobutene dimerization and the fluidzed system are both obtained. It is a particular feature of the present invention that a method of suppressing (or controlling) higher isobutene polymerization (e.g., trimer, tetramers and copolymers) in a fixed catalyst bed has been obtained. It is also a feature of the present invention that temperature control of the fixed bed dimerization has been obtained. These and other advantages and features will be more apparent from the further discussion of the invention.